Nephelometric analysis is a well-known method of quantitative analysis in which the concentration of suspended matter in a liquid is determined by optical means. The method is based upon a comparison of the intensity of light scattered by a sample under defined conditions with the intensity of light scattered by a standard reference suspension. The higher the intensity of the scattered light is, the higher the turbidity.
Utilizing a nephelometer with a light source for illuminating the sample and at least one photoelectric detector with a readout device to indicate the intensity of the light scattered at right angles to the path of the incident light, a turbidimeter is usually designed to meet the specifications set forth in the EPA Guidance Manual entitled, “Method 180.1, Determination of Turbidity by Nephelometry”. This manual specifies, among other things, that the turbidimeter be so designed that little stray light reaches the detector in the absence of turbidity. Instruments which are sufficiently sensitive to permit detection of a turbidity difference of 0.02 NTU or less in waters having turbidities less than 1 unit are considered to have met this criterion.
To this end, those previously proposing flow-through sample cells for use with a dynamic (continuously flowing) sample in an on-line version of the turbidimeter have kept the well of its cell port the same as if this instrument were being used to measure the turbidity of a static (non-flowing) sample of fixed volume. Only the sample cell and one or more of its inlet and outlet ports have been modified to accommodate a dynamic sample. In U.S. Pat. No. 5,475,486, for example, Paoli provides a standpipe which is connected to and extends upwardly from the bottom outlet port of his flow-through sample cell to a point above the top of the sample cell. Prior to use, the entire sample cell and the standpipe for most of its length are slideably inserted into the well of the cell port of a standard laboratory turbidimeter.
Further specifications as set forth in the EPA Guidance Manual describe procedures, using primary and secondary standard suspensions, to calibrate turbidimeters and the frequency with which their calibration should be verified. Unfortunately, in the case of prior art on-line turbidimeters, substantial quantities of very costly reagent must be used with each initial and subsequent primary calibration. A common approach is to use 1 liter sample reservoirs to purge flow-through sample cells, especially those having a single top discharge port. But even with Paoli's standpipe-augmented sample cell, reagent well in excess of that which would be required for the sample cell alone must be consumed because the standpipe siphons liquid away from the sample cell whenever one either partially fills the cell in order to flush any residual liquid from it or subsequently fills the cell sufficiently to actually calibrate it.
On the other hand, dead spaces in prior art sample cells which lack a bottom outlet port tend to collect settleable contaminants from the sample flow stream. These contaminants can then reenter the flow stream as a blob, giving rise to erroneous turbidity readings.
For use in a process such as water purification, it is highly desirable to utilize on-line turbidimeters capable of giving accurate real-time readings of the turbidity of process waters.